U.S. Pat. No. 3,470,021 (Hendricx et al.) says that in high speed recording on a magnetic recording medium (which may be flexible backing "friction causes lumps of redeposited debris of the magnetic recording layer to become attached to the surface of the recording material and/or to the heads" (col. 1, lines 28-31), but that good resistance to such lump formation is provided by incorporating oleic acid into the recording layer. It further says that the "carboxylic acid group of the oleic acid has a greater affinity to the magneticaly susceptible particles than for the binder . . . and consequently it is oriented towards the surface of those particles. The fatty or oleophilic part of the oleic acid, however, tends to turn away from the particles and in this way brings about an excellent lubrication, whereby the resistance of the magnetic recording material . . . to lump formation and abrasion is considerably improved" (col. 1, lines 60-72). The oleic acid is said to be useful in amounts ranging from 3 to 10 percent of the total weight of binder, magnetically susceptible particles, and oleic acid. In Example 1 of U.S. Pat. No. 3,630,772 (Seidel), the recording layer of a flexible magnetic recording medium includes 6.3% of oleic acid based on the weight of magnetically susceptible particles, namely needle-shaped gamma-iron oxide.
U.S. Pat. No. 4,436,786 (Ohkawa et al.) says that it has been conventional practice to employ various lubricants such as higher fatty acids or their derivatives (also naming other classes of materials as lubricants) but that "none of these lubricants can provide adequate lubricity to the magnetic tape" (col. 1, lines 39-46). It indicates that either silicone oils or higher fatty acids may exude onto the surface of the tape if used in large amounts, e.g., when the fatty acid is 6% by weight or more (col. 3, lines 2-15) "on the basis of a binder" (col. 3, line 43).
While the only magnetically susceptible particles mentioned in the Hendricx and Ohkawa patents are iron oxides, U.S. Pat. No. 4,465,737 (Miyatuka et al.) specifically concerns flexible magnetic recording media based on ferromagnetic metal powders that "are used to increase the magnetic recording density and output level and provide more saturation magnetization and coercivity" (col. 1, lines 18-22). It indicates that such a so-called "metal tape" is necessary for miniaturized videotape recorders that require higher C/N (carrier-to-noise) ratios than those of media based on oxide particles as in the previously cited patents. It indicates that to achieve higher recording density, the spacing loss (tape-head clearance) should be smaller, obtainable from a smoother tape surface, but that a smooth surface results in poor runability due to high contact resistance, making "the tape abrasive which causes problems in the durability of tapes" (col. 1, lines 49-61). The answer to this is "a magnetic layer containing a ferromagnetic metal powder having not less than 30 m.sup.2 /g of specific surface area, and a fatty acid and a fatty acid ester in an amount of 2 to 10 wt% based on the ferromagnetic metal powder, in which the amount of fatty acid ester is 15 to 60 wt% based on the total amount of the fatty acid and fatty acid ester, and having a surface roughness of not more than 0.03 micrometers" (col. 2, lines 24-31). Useful fatty acids include palmitic, stearic, myristic, lauric and oleic acids (col. 3, lines 53-57). Each of Examples 1-3 have the same binder which includes 2 parts of fatty acid (oleic plus palmitic) and 1-1/3 part of a fatty acid ester per 100 parts by weight of ferromagnetic powder. In the only use of more than 2 parts of fatty acid reported in the patent, (Comparative Example 4), a total of 2-2/3 parts of the same two fatty acids were used, based on 100 parts ferromagnetic powder, together with only 1/3 part of the same fatty acid ester (i.e., less than the 15% mentioned above). This produced head clogging.
U.S. Pat. No. 4,439,486 (Yamada et al.) concerns problems in flexible magnetic recording media from using fine ferromagnetic particles which, like those of the Miyatuka Patent, have a specific surface area of at least 30 m.sup.2 /g, but the only fine ferromagnetic particles mentioned in the Yamada patent are oxides, as opposed to Miyatuka's metal particles. As in the Miyatuka patent, the Yamada patent indicates that fine particles involve a problem of wear resistance. It addresses that problem by a modified binder. The binder "may contain optional additives such as a dispersant, lubricant ..." (paragraph bridging columns 4 and 5). "Suitable dispersants include aliphatic acids having 12 to 18 carbon atoms ... such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linolic acid, linolenic acid and stearoic acid" (col. 5, lines 3-18). "The dispersants mentioned above can also be used as a lubricant. Other suitable lubricants are silicone oils .." (col. 5, lines 19-21). The only specific binder composition employs 1 part of myristic acid and 0.5 part of butyl stearate per 100 parts by weight of ferromagnetic particles (col. 8. lines 16-17).
Japanese Patent Application JA58-31488 filed Feb. 25, 1983 (Miyata et al.) is concerned with improving both the magnetic properties and the durability of flexible magnetic recording media and indicates that this is achieved by incorporating a trialkoxy silane into certain binders. In Example 1, the recording layer comprises by weight 100 parts of cobalt-containing gamma-Fe.sub.2 O.sub.3 particles, 5 parts of myristic acid, and 10 parts of n-butyl stearate, but nothing more is said about those materials or their utility. While the application names various ferromagnetic oxide and metal particles as being useful, it says nothing about their sizes or surface area.
Since the cost of ferromagnetic oxide particles is far less than that of ferromagnetic metal particles, it would be highly desirable to be able to use oxide particles to achieve objectives such as those of the Miyatuka patent, e.g., to obtain the high carrier-to-noise ratios required by miniaturized videotape recorders while also attaining the stable running properties and better durability to which that patent is directed. However, a high loading is required to achieve a high carrier-to-noise ratio and also the ultra-smooth surface that is needed to attain a high carrier-to-noise. Such ultra-smooth surfaces have resulted in high friction between the tape and head surfaces, resulting in poor runability and poor resistance to abrasion, problems faced in the Miyatuka patent when using fine ferromagnetic metal particles. Although fine ferromagnetic oxide particles have been available for some time, it is believed that the absence of a solution to those problems has deterred anyone from using such particles in magnetic recording tapes on a commercial basis.